Small to medium varieties include Bednall Beauty, which grow up to 2 feet (61 cm), and Bishop of Llandaff, which grow up to 3 feet (0. 91 m). Larger varieties include Hadrian’s Sunlight and Twyning’s After Eight, which grow 4 feet (1. 2 m) or higher. [2] X Trustworthy Source Royal Horticultural Society Leading gardening charity in the U. K. providing resources for identifying, growing and caring for flowers and other plants Go to source A single tuber needs about 1 to 2 feet (30 to 61 cm) of space, so you’ll probably only need 1 per container. [3] X Research source

For the largest varieties, go with a 24 by 24 inch (61 by 61 cm) pot, which usually holds around 25 gallons (95 L) of soil.

Plastic containers are lightweight, so go for sturdy ceramic or terracotta pots instead. Keep in mind terracotta and unglazed ceramic pots absorb water, so they can freeze and crack if they’re left out during the winter. [6] X Research source

Dahlias grow best in well-drained environments. While they require lots of water, their fine roots are prone to rotting if the soil is too waterlogged. Fill the bottom with 2 inches (5. 1 cm) of gravel for improved drainage.

Be sure to thoroughly rinse away all soap residue. It’s particularly important to wash the pot if you’ve used it for other plants in the past.

Note that garden soil isn’t suitable for container plants. Be sure you purchase potting soil labeled for potted plants.

If your area is prone to frost in the spring and you don’t want to wait to plant, start your dahlias indoors, then move them outside.

Eventually, you’ll plant your dahlia tuber about 4 to 6 inches (10 to 15 cm) deep, and you’ll want to leave 1 inch (2. 5 cm) between the surface of the soil and the rim of the pot at the end of the planting process. To meet these measurements, start by adding soil to within 5 to 7 inches (13 to 18 cm) of the rim.

Any fertilizers recommended for flowering plants or vegetables are suitable for dahlias. You might find store-bought fertilizers with N-P-K numbers, which stand for nitrogen, phosphorus, and potassium. Good choices for dahlias include 5-10-15, 5-10-10, and 10-20-20 fertilizers. Use a fertilizer with a higher nitrogen value earlier in the season and reduce it over time. Read your product’s instructions, and use it as directed.

Insert the stake in the soil next to where you’ll place the tuber. Choose a stake that will touch the bottom of the pot and extend at least 3 feet (0. 91 m) above the soil’s surface. For a 12 inch (30 cm) pot, go with a 4 foot (1. 2 m) stake. Tie the stem to the stake loosely as it grows.

The eye looks like a bud, and it’s located on the opposite side of the tuber’s roots.

You don’t want the soil to dry out completely, but the young tuber will rot if you soak it. Unless you live in a hot climate, you might not need to water at all until the first shoots and leaves emerge from the soil. [18] X Research source

As the stalk grows, continue adding small amounts of soil until 1 inch (2. 5 cm) of empty space remains between the top of the soil and the rim of the pot. Dahlias grow quickly, and you’ll see substantial foliage within 2 weeks. Blooms typically appear about 8 weeks after planting.

If you keep your dahlia indoors, you’ll need to supplement sunlight with fluorescent plant lights. You’ll also need supplemental lighting if you’re starting your plant indoors in early spring. Set the light about 6 inches (15 cm) above the top rim of the pot for newly planted tubers and raise the light as the plants grow.

Higher amounts of phosphorus promote blood production, and potassium encourages root development. Apply your product according to its label’s instructions.

Pinch back the tips before flower buds form when the plant is about 16 inches (0. 41 m) tall to encourage a bushier growth. Removing the bottom leaves improves air circulation, which reduces the risk of mildew.

Find a chemical insecticide and miticide labelled for flowering plants at your local garden center. Apply your product as directed. Use organic insecticides, such as horticultural soap or neem oil, for a more natural approach.

Store the tuber in a dry, dark place that’s 40 to 50 °F (4 to 10 °C) during the winter, then replant it in the spring.